Genesis of apocarbonatitic titanium metasomatites of the Petyayan-Vara rare-earth occurrence (Vuoriyarvi, the Kola region)

The objects of the study are apocarbonatitic titanium metasomatites ("titanium carbonatites") associated with the rare earth carbonatites of the Petyayan-Vara area of the Vuoriyarvi complex (the Kola region). In this paper, the following mechanism for the formation of these rocks has been substantiated based on the agreed results of mineralogical and geochemical studies. Prior to the onset of carbonatite genesis, a fluorine-enriched fluid phase originated in the lower horizons of the complex passed along the deep-permeating fracture system of several hundred meters length up to the level of the modern erosion surface. It transported Al, Fe2+, Mg, Ti, P into the pyroxenites and Si, Ca and Na out of them, as a result of which the pyroxenites were transformed into giant-grained phlogopite rocks - glimmerites. The most probable source of this fluid is alkaline aluminosilicate magma. Then carbonate melts intruded along the same fractures. In the course of carbonatite genesis, F-fluid caused a local migration of K, Al, Si, Fe, P, Ti, Nb, Ta, Zr, Hf and HREE out of glimmerites into igneous dolomite carbonatites, which led to the formation of apocarbonatitic titanium metasomatites. They represent several paragenetic associations superimposed on each other, the mineral composition and the formation sequence of which correspond to the metasomatic column zones directly observed within the contact "carbonatite - altered pyroxenite". The separation of HFSE and REE is controlled by the same metasomatic column: Ti, Nb and Ta were accumulated in titanium carbonatites, i. e. in associations of the frontal and intermediate zones, and Zr, Hf and HREE - in apatitized fields corresponding to the rear zone of the column. Accordingly, the fractionation of these elements occurred due to the "fluid - rock" interaction. Subsequently, the same long-lived fractures served as a channel for REE-Sr-Ba-S fluids, but the recrystallization caused by K-Al-Si-Ti-F-metasomatism made titanium carbonatites dense and fine-grained in texture, what, in most cases, "protected" these rocks from the influences of later processes.